The present invention relates to a method for demolding a safety-seal strip of a closure cap. The invention further relates to an injection-molding apparatus designed to permit the demolding method to be used as part of a production cycle.
A method for molding a safety-seal strip of a closure cap is known from French patent 2,426,617. Reference is made to this publication for an explanation of the technical background of the invention.
Plastic closure caps suitable for closing beverage bottles or similar containers have found wide use. Such closure caps often comprise an upper disk onto whose underside a seal ring is molded, a cylindrical side wall which on its inner surface is provided with a screw thread, and a safety-seal strip suspended from the lower rim of the closure cap by relatively weak webs distributed over the periphery. When such a closure cap is screwed or pressed onto the neck of a beverage bottle, thus closing the bottle securely, the safety-seal strip serves to indicate that the closure is intact. For this purpose, a bead on the inner circumference of the safety-seal strip engages a corresponding groove in the neck of the bottle. When the bottle is opened for the first time, the safety-seal strip is completely or partly torn off the closure cap by the unscrewing motion.
The manufacture of a closure cap with a safety-seal strip suspended therefrom is not simple since the injection mold comprises not only several undercuts but also an intended break line for the subsequent use. The basic problem in the manufacture of such safety-seal strips is that they must not tear off during the manufacturing process, whereas later, in actual use, they should not offer appreciable resistance when the closure cap is unscrewed from the bottle. Since the shape of the mold core of the injection-molding apparatus employed in the manufacture is quite similar to that of the bottle neck used later, provision must be incorporated in the injection-molding apparatus for freeing the safety-seal strip intact. Such provision involves in particular the demolding phase, in which a finished injection-molded part is removed from the mold. Within that phase, the steps during which the safety-seal strip is stripped from the core are particularly critical.
The injection-molding apparatus disclosed in French patent 2,426,617 and the respective sequence of operations in the manufacture offer the following solution of the basic problem outlined: The closed injection mold essentially consists of an outer cavity forming member which determines the outer shape of the closure cap, and of a core which establishes the inner shape of the closure cap. The outer mold cavity overall includes, in addition to the upper cover, an injection crown which injects plastic material at the level of the webs (in other words, at the level of the parting line between closure cap and safety-seal strip), and a mold ring which molds the lower outer surface of the safety-seal strip. On completion of the injection-molding operation, the upper cover plate can be taken off and the injection crown can be used as a stripper plate. In this prior-art solution, the mold ring, located under the stripper plate, is stationary. During the stripping operation, a clearance is thus created at the lower outer surface of the safety-seal strip between the stationary mold ring and the upwardly carried crown, and in that clearance the safety-seal strip is able to distend outwardly.
In the prior-art solution, the mold core comprises an axially displaceable cylinder which molds both the screw thread on the inner surface of the closure cap and an undulating circumferential bead on the inner surface of the safety-seal strip. This movable internal mold is carried along upwardly during the demolding phase until it strikes a stop. At that instant, both the closure cap and the safety-seal strip can be distended into the externally created clearances; only in the area between the webs does the crown, as the stripper plate, engage the underside of the closure cap. At the same time, the crown engages the underside of a small undercut molded into the safety-seal strip in the upper portion of its outer surface. It is on these two peripheral lines that the forces which strip the whole closure cap from the mold core, now locked in position, are acting simultaneously. The connecting webs therefore are not subjected to any axial stresses. After stripping, the finished part is ejected, for example, by compressed air. Further details on the injection molding of closure caps provided with safety-seal strips in general and on the prior-art demolding of a finished injection-molded part in particular will be found in French patent 2,426,617.
In the prior-art method, it has proved onerous and mechanically complicated that in demolding a movable mold cylinder which is guided between two stationary cylinders, and which defines the inner surface of the closure cap and the safety-seal strip with all undercuts, has to participate in part of the stripping motion before it is locked in position to permit the molded part to be definitely removed.